Re: [PATCH RFC v3 4/9] mm: Export alloc_contig_range() / free_contig_range()

From: Michal Hocko
Date: Wed Oct 16 2019 - 07:20:56 EST

On Thu 19-09-19 16:22:23, David Hildenbrand wrote:
> A virtio-mem device wants to allocate memory from the memory region it
> manages in order to unplug it in the hypervisor - similar to
> a balloon driver. Also, it might want to plug previously unplugged
> (allocated) memory and give it back to Linux. alloc_contig_range() /
> free_contig_range() seem to be the perfect interface for this task.
> In contrast to existing balloon devices, a virtio-mem device operates
> on bigger chunks (e.g., 4MB) and only on physical memory it manages. It
> tracks which chunks (subblocks) are still plugged, so it can go ahead
> and try to alloc_contig_range()+unplug them on unplug request, or
> plug+free_contig_range() unplugged chunks on plug requests.
> A virtio-mem device will use alloc_contig_range() / free_contig_range()
> only on ranges that belong to the same node/zone in at least
> MAX(MAX_ORDER - 1, pageblock_order) order granularity - e.g., 4MB on
> x86-64. The virtio-mem device added that memory, so the memory
> exists and does not contain any holes. virtio-mem will only try to allocate
> on ZONE_NORMAL, never on ZONE_MOVABLE, just like when allocating
> gigantic pages (we don't put unmovable data into the movable zone).

Is there any real reason to export as GPL rather than generic
EXPORT_SYMBOL? In other words do we need to restrict the usage this
interface only to GPL modules and why if so. All other allocator APIs
are EXPORT_SYMBOL so there should better be a good reason for this one
to differ. I can understand that this one is slightly different by
requesting a specific range of the memory but it is still under a full
control of the core MM to say no.

> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: Michal Hocko <mhocko@xxxxxxxx>
> Cc: Vlastimil Babka <vbabka@xxxxxxx>
> Cc: Oscar Salvador <osalvador@xxxxxxx>
> Cc: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
> Cc: Mike Rapoport <rppt@xxxxxxxxxxxxx>
> Cc: Dan Williams <dan.j.williams@xxxxxxxxx>
> Cc: Alexander Duyck <alexander.h.duyck@xxxxxxxxxxxxxxx>
> Cc: Pavel Tatashin <pavel.tatashin@xxxxxxxxxxxxx>
> Cc: Alexander Potapenko <glider@xxxxxxxxxx>
> Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>

Other than that, I do not think exporting this function is harmful. It
would be worse to reinvent it and do it wrong.

I usually prefer to add a caller in the same patch, though, because it
makes the usage explicit and clear.

Acked-by: Michal Hocko <mhocko@xxxxxxxx> # to export contig range allocator API

> ---
> mm/page_alloc.c | 2 ++
> 1 file changed, 2 insertions(+)
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 3334a769eb91..d5d7944954b3 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -8469,6 +8469,7 @@ int alloc_contig_range(unsigned long start, unsigned long end,
> pfn_max_align_up(end), migratetype);
> return ret;
> }
> +EXPORT_SYMBOL_GPL(alloc_contig_range);
> #endif /* CONFIG_CONTIG_ALLOC */
> void free_contig_range(unsigned long pfn, unsigned int nr_pages)
> @@ -8483,6 +8484,7 @@ void free_contig_range(unsigned long pfn, unsigned int nr_pages)
> }
> WARN(count != 0, "%d pages are still in use!\n", count);
> }
> +EXPORT_SYMBOL_GPL(free_contig_range);
> /*
> --
> 2.21.0

Michal Hocko